Trac has some unusual or better said special conventions about default URL paths (or URL endpoints). These are specified relative to base Trac URL.

• /chrome - the handler for this URL just serves files that are placed in htdocs folder of Trac environment.
  
  These files are usually images, .css and .js that are called static resources, because they don't require any server-side processing and can be served as-is. In fact to avoid firing Trac request handling many people prefer setups when static resources are served by web server directly.

• /chrome/site - using this URL Trac accesses static resources for TracInterfaceCustomization that were uploaded to environment's htdocs/ directory
• /chrome/common - with this URL Trac accesses static resources used by default Trac scheme - these are usually found in htdocs/ subdirectory in Trac installation location

• /chrome/<pluginname> - plugins may also have static resources that can be accessed using Trac chrome handler

All static resources can be exported with trac-admin deploy command to serve them independently. The process is described in detail on TracCgi#MappingStaticResources page. Just do not forget to re-export them after installing new plugins or upgrading Trac.